Re: [PATCH bpf-next] tools/resolve_btfids: fix cross-compilation to non-host endianness

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Jan 26, 2024 at 03:40:11PM -0800, Andrii Nakryiko wrote:
> On Tue, Jan 23, 2024 at 4:08 AM Viktor Malik <vmalik@redhat.com> wrote:
> >
> > The .BTF_ids section is pre-filled with zeroed BTF ID entries during the
> > build and afterwards patched by resolve_btfids with correct values.
> > Since resolve_btfids always writes in host-native endianness, it relies
> > on libelf to do the translation when the target ELF is cross-compiled to
> > a different endianness (this was introduced in commit 61e8aeda9398
> > ("bpf: Fix libelf endian handling in resolv_btfids")).
> >
> > Unfortunately, the translation will corrupt the flags fields of SET8
> > entries because these were written during vmlinux compilation and are in
> > the correct endianness already. This will lead to numerous selftests
> > failures such as:
> >
> >     $ sudo ./test_verifier 502 502
> >     #502/p sleepable fentry accept FAIL
> >     Failed to load prog 'Invalid argument'!
> >     bpf_fentry_test1 is not sleepable
> >     verification time 34 usec
> >     stack depth 0
> >     processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >     Summary: 0 PASSED, 0 SKIPPED, 1 FAILED

hum, I'd think we should have hit such bug long time ago.. set8 is
there for some time already.. nice ;-)

> >
> > Since it's not possible to instruct libelf to translate just certain
> > values, let's manually bswap the flags in resolve_btfids when needed, so
> > that libelf then translates everything correctly.
> >
> > Fixes: ef2c6f370a63 ("tools/resolve_btfids: Add support for 8-byte BTF sets")
> > Signed-off-by: Viktor Malik <vmalik@redhat.com>
> > ---
> >  tools/bpf/resolve_btfids/main.c | 35 +++++++++++++++++++++++++++++++--
> >  1 file changed, 33 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> > index 27a23196d58e..440d3d066ce4 100644
> > --- a/tools/bpf/resolve_btfids/main.c
> > +++ b/tools/bpf/resolve_btfids/main.c
> > @@ -646,18 +646,31 @@ static int cmp_id(const void *pa, const void *pb)
> >         return *a - *b;
> >  }
> >
> > +static int need_bswap(int elf_byte_order)
> > +{
> > +       return __BYTE_ORDER == __LITTLE_ENDIAN && elf_byte_order != ELFDATA2LSB ||
> > +              __BYTE_ORDER == __BIG_ENDIAN && elf_byte_order != ELFDATA2MSB;
> 
> return (__BYTE_ORDER == __LITTLE_ENDIAN) != (elf_byte_order == ELFDATA2LSB);
> 
> ?
> 
> > +}
> > +
> >  static int sets_patch(struct object *obj)
> >  {
> >         Elf_Data *data = obj->efile.idlist;
> >         int *ptr = data->d_buf;
> >         struct rb_node *next;
> > +       GElf_Ehdr ehdr;
> > +
> > +       if (gelf_getehdr(obj->efile.elf, &ehdr) == NULL) {
> > +               pr_err("FAILED cannot get ELF header: %s\n",
> > +                       elf_errmsg(-1));
> > +               return -1;
> > +       }
> 
> calculate needs_bswap() once here?
> 
> >
> >         next = rb_first(&obj->sets);
> >         while (next) {
> > -               unsigned long addr, idx;
> > +               unsigned long addr, idx, flags;
> >                 struct btf_id *id;
> >                 int *base;
> > -               int cnt;
> > +               int cnt, i;
> >
> >                 id   = rb_entry(next, struct btf_id, rb_node);
> >                 addr = id->addr[0];
> > @@ -679,6 +692,24 @@ static int sets_patch(struct object *obj)
> >
> >                 qsort(base, cnt, id->is_set8 ? sizeof(uint64_t) : sizeof(int), cmp_id);
> >
> > +               /*
> > +                * When ELF endianness does not match endianness of the host,
> > +                * libelf will do the translation when updating the ELF. This,
> > +                * however, corrupts SET8 flags which are already in the target
> > +                * endianness. So, let's bswap them to the host endianness and
> > +                * libelf will then correctly translate everything.
> > +                */
> > +               if (id->is_set8 && need_bswap(ehdr.e_ident[EI_DATA])) {
> > +                       for (i = 0; i < cnt; i++) {
> > +                               /*
> > +                                * header and entries are 8-byte, flags is the
> > +                                * second half of an entry
> > +                                */
> > +                               flags = idx + (i + 1) * 2 + 1;
> > +                               ptr[flags] = bswap_32(ptr[flags]);
> 
> we are dealing with struct btf_id_set8, right? Can't we #include
> include/linux/btf_ids.h and use that type for all these offset
> calculations?..

we could, there's tools/include/linux/btf_ids.h, which we could include
in here, we do that in selftests.. but it needs to be updated with latest
kernel updates (at least with set8 struct)

> 
> I have the same question for existing code, tbh, so maybe there was
> some good reason, not sure...

I think the test came later and I did not think of it for the resolve_btfids
itself, I guess it might make the code more readable

thanks,
jirka

> 
> > +                       }
> > +               }
> > +
> >                 next = rb_next(next);
> >         }
> >         return 0;
> > --
> > 2.43.0
> >
> >